[Spice-devel] [virtio-dev] Re: [PATCH v2 0/1] VirtIO video device specification

[Keiichi Watanabe]

Hi,

On Tue, Jan 21, 2020 at 3:44 PM Gerd Hoffmann <kraxel at redhat.com> wrote:
>
> > > > Can't this problem be solved by adding "offset" field in virtio_video_mem_entry?
> > > >
> > > > struct virtio_video_mem_entry {
> > > >   le64 addr;
> > > >   le32 length;
> > > >   le32 offset;
> > > >   u8 padding[4];
> > > > };
> > > >
> > > > Here, "addr" must be the same in every mem_entry for (1) hardware.
> > >
> > > No.  virtio_video_mem_entry is basically a scatter list entry, you use
> > > an *array* of these entries to describe your buffer (unless you are
> > > using CMA due to hardware requirements, in this special case you have
> > > only one entry in your array).
> >
> > I see. I forgot about scatter list.
> > However, I'm still not sure about the usage for CMA.
> > if we're using CMA for a multiplanar format, how can the device know
> > where the second plane start from?
> > In my understanding, the number of entries in this case should be the
> > same with the number of planes and
> > "entries[0].addr + entries[0].length == entries[1].addr" should hold.
>
> With the one-buffer-per-frame model you could add a plane_offsets[4]
> field to virtio_video_resource_create.
>
> The virtio_video_mem_entry array describes the whole buffer for all
> planes, the plane_offsets array says where the individual planes start
> inside the buffer.

Got it. It makes sense to have plane_offsets[] separately.
Thanks!

Best regards,
Keiichi

>
> cheers,
>   Gerd
>
>
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